Cellular radiotelephones have become increasingly popular for many different reasons, including their potential usage in case of emergency. Cellular radiotelephones adequately meet the needs of emergency situations because they can be carried with a user to be readily available whenever and wherever needed. However, cellular radiotelephones have a distinct disadvantage from conventional landline telephones in that they may not provide sufficient information in which to reliably perform a return call.
When an emergency call is placed from a cellular radiotelephone, the call is routed to a public safety answering position (PSAP). If the two parties are unexpectedly disconnected or if the call has terminated but additional information is needed, a "call-back" may be desired. A call-back is a return call made by the PSAP directed to the cellular radiotelephone from which the emergency call originates.
In accordance with conventional methodologies, when the cellular radiotelephone is a subscriber within it's own service area, a call-back is a local call to the radiotelephone Mobile Identity Number (MIN) for the cellular radiotelephone making the original emergency call. Likewise, the subscriber can be a roamer who has activated a "call follow" service. In conventional systems, a call follow service assigns a temporary local phone number to the roaming cellular radiotelephone and sends a call forward order to the roamer's service area. When an emergency call is placed by the roaming cellular radiotelephone, the PSAP receives the local phone number as the number from where the emergency call originated. During call-back, the PSAP directs the call to the local phone number. The roaming area MTSO receives the call-back to the local phone number and translates the temporary local phone number into the roaming cellular radiotelephone's MIN. The call-back is then directed to the roaming cellular radiotelephone.
If an emergency call is originated by a subscriber who is roaming and has not activated a call follow service, the PSAP will receive a MIN that is invalid in the roamer's current service area. In accordance with conventional methodologies, a call-back will fail because a call-back to an invalid MIN will not be placed through a MTSO.
In these situations, a call-back is not necessarily successful. A further complication arises if the cellular radiotelephone switches to the non-preferred A-side or B-side system after the emergency call but prior to the attempted call-back. This can occur if the cellular radiotelephone moves in an area were the signal strength of the non-preferred system is significantly greater than that of the preferred system, or if the preferred system becomes unavailable. In accordance with conventional methodologies, call-backs are directed through the preferred A-side or B-side system for that cellular radiotelephone, hence the call-back will fail because the cellular radiotelephone is no longer monitoring that cellular system.
If a call-back is not received by the cellular radiotelephone, the PSAP could possibly attempt the call-back over the other one of the local A-side or B-side systems to ameliorate the situation where the cellular radiotelephone switches systems. In accordance with current methodologies, call-back attempts over both A-side and B-side MTSOs would be performed sequentially thereby costing valuable time. Moreover, if the caller is roaming and the caller's home service area has no roaming agreement with either system in a roaming area, the call-back attempt will still fail.
Additional problems arise with "unauthorized" cellular radiotelephones. Unauthorized cellular radiotelephones are those cellular radiotelephones that do not have a valid, unique MIN. While normal calls cannot originate from an unauthorized cellular radiotelephone, emergency calls are an exception. Emergency calls are routed to the PSAP without confirming the validity of the cellular radiotelephone to be using the system. However, current methodologies do not allow for call-backs to be successfully made to these unauthorized cellular radiotelephones. Unauthorized cellular radiotelephones include "non-subscribers" and "inactive" cellular radiotelephones.
A non-subscriber is a cellular radiotelephone that was once a subscriber with an assigned MIN, but is no longer a paying customer. The MIN may remain programmed in the cellular radiotelephone, however it is invalid because the MIN is no longer assigned to that cellular radiotelephone by the home service area MTSO. A normal call can only be connected if the MIN and the radiotelephone Electronic Serial Number (ESN) match the assigned MIN/ESN, as reported by the home service area MTSO. Unless both numbers match, the home service area for that MIN will not steer the call to this unauthorized cellular radiotelephone.
An inactive cellular radiotelephone has never been a subscriber to a cellular service, therefore it may not have a unique MIN. A call-back cannot be performed because this cellular radiotelephone has no dialable telephone number.
Given these examples, if an emergency call originating from a cellular radiotelephone is disconnected, conventional Mobile Telephone Switching Offices (MTSOs) may be unable to set up a call-back from the PSAP to the cellular radiotelephone. This inability to return a call is a serious problem in an emergency situation.